We are not mean = chúng tôi không sấu tính

We are above average....

Is "xấu tính" similar to "khó tính"? Both seem to me to mean something similar like "bad" or "hard" personality (tính). I mention "khó tính" because that's what was described in another Vietnamese class as the closest meaning to the adjective "mean"

why is that we can just use không here instead of không phải là?